Resist etching



United States Patent RESIST ETCHING Steve Eisner, Detroit, Mich assignor to Ford Motor Company, Dearborn, Mich., a corporation of Delaware No Drawing. Application November 13, 1956 Serial No. 621,505

3, Claims. (Cl. 41-.43)

This application relates to novel compositions and processes for the precision etching of acid resistant metals par ticularly the stainless steels.

Chemicalforming of metals is carried out by partially masking the piece to be etched with a resistant coating then submerging the piece in a corrosive medium which removes the metal in the unprotected areas. To successfully apply the process to the formation of highly complex configurations or intricate designs having close tolerances requires that the corrosive medium attack the metal uniformly, at a constant rate, and at a rate rapid enough to be practical. At the same time the resistant coating must form a continuous film on the metal which adheres tenaciously during the etch, which is impervious to the corrosive medium, and which is easily stripped off the metal after the etching is complete.

While certain etching solutions and resists have long been known and used in the etching of copper and mild steels, methods of etching the stainless steels have been unsuccessful because the strong acids required to etch the steel also attacked and loosened the resist coating so that the etch pattern could not be maintained. Furthermore, many strong acids attack the steel unevenly forming rough and pitted surfaces while others form a passive coating on the steel so that the rate of reaction becomes very slow after the initial attack.

Having these defects in mind it is an object of this invention to provide resist compositions for the etching of corrosion resistant steels which compositions withstand the corrosive action of the acid solution used to etch the steel and may be precisely deposited upon the steel so that a clear sharp etch pattern is obtained.

Another object is to provide an acid solution which attacks stainless steel quickly and uniformly and a method of employing this solution as a precision etch bath.

Concentrated sulfuric acid rapidly attacks stainless steels at elevated temperatures, however, the attack is too violent for close control and is not even over the exposed surface but leaves a very rough surface at the etched portions. If the temperature of the acid is lowered it is found that the rate of attack is decreased to controllable limits but ceases altogether after a short time due to the formation of a passive coating on the metal which inhibits further reaction. While this acid is suitable for use in the process it is not the preferred acid because the control of the process is difiicult.

Acids such as phosphoric acid appear to attack the resist coating and loosen it from the metal when the acids are heated to a temperature at which they attack the steel. The demarcation between the etched and unetched surface is therefore not sharp and the etch pattern is rather blurred and uneven.

A concentrated solution of hydrochloric acid is a preferred etchant. It attacks the stainless steels rapidly but uniformly and does not form a passive coating upon prolonged contact with the metal. For example, a bath comprising a solution of about 37% hydrochloric acid at a temperature of about 67 C., was found to erode 430 stainless steel at a lineal rate of .0375 inch per hour. Examination of the etched surface showed it to be smooth and free from objectionable pits.

Cured epoxy type resins resist attack by hydrochloric acid and are useful as a resist coating. These resins make excellent resists not only because-of their acid resistant properties but also because they adhere well to the steel during the etching and yet are easily stripped 01f the completed etched piece. On the otherhand, resins of the polyvinylchloride or nitrocellulose types do not adhere as tenaciously to the metal nor are they as resistant to the strong acids which mustbe usedin order to corrode the metal.

It is advisable to add small percentages of an activator to the epoxy resin to promote cross linkage in the cured polymer. Activator-s such as pyridine, diethyl amine, piperidine or diethylene triamine, which are commonly used to catalyze the heat curing of the epoxy resins; all appear to be satisfactory. The resin is cured by heating to temperatures of about F., to 450 F, for periods which vary from one minute to about two hours, depending on the cure temperature;

The resist may be deposited upon theworkpiece' by any convenient method, however, for the more intricate patterns and close tolerance work it is preferable to use a modified silk screen process for depositing the resist pattern on the metal and then to cure the resin in situ. Conventional silk screen techniques are employed, that is the acid resist is squeegeed onto the metal through a silk screen having portions of its mesh plugged or stopped off in the form of the pattern by processes known in the art. Small percentages of a suitable solvent such as xylene or tolulene are added to the uncured epoxy to increase its fiowability during the coating process. It was found that in order to adapt the silk screen process for use in depositing the epoxy onto the steel, it is necessary to pre coat the silk screen with a solution of a suitable silicone to prevent formation of objectionable streamers of the resin caused by adhesion of the resin to the silk screen pattern. By eliminating these streamers it is possible to produce a resist pattern on the metal which has sharp, clear edges.

When etching the pattern it is desirable to place the metal in the bath so that the plane of the pattern is substantially parallel to the surface of the bath. If the pattern is normal to the etchant surface, the hydrogen bubbles evolved by the reaction tend to agitate the solution unevenly and thereby promote localized overetching. The reasons for this is that the lines of the pattern which are approximately normal to the surface tend to channel the bubbles to the liquid surface thereby agitating the corrosive bath adjacent to these lines While the more nearly horizontal lines prevent this channeling effect. Maintain ing the plane of the etch pattern parallel to the surface eliminates these effects.

Example A silk screen pattern treated with a GE. SRSI silicone was placed against the metal to be etched. An epoxy resin having about 225-290 grams of resin per gram equivalent of epoxide and containing about 10% methylethyl ketone solvent, 1% GE. SR82 silicone to prevent balling up of the resin as it lay on the steel and a small percentage of diethylene triamine was employed as the resist composition. This was squeegeed on apiece of 430 stainless steel through the screen and was then heated to volatilize the solvent and to cure the epoxide coating. The metal carrying the epoxy resin mask was then placed in the etch bath of concentrated hydrochloric acid at 67 C. The plane of the surface to be etched was positioned parallel to the liquid surface to minimize the effect of the geometry of the etch pattern on the agitation caused by the evolution of hydrogen. After the metal was cut to the desired depth, it was removed from the bath and the resist was stripped off. The finished piece was observed to have an extremely accurate etch pattern with a minimum of side etch.

It will be understood that the invention is not to be limited to the exact examples shown anddescribed, but that various changes and modifications may be made without departing from the spirit and scope of the invention, as defined in the appended claims.

I claim:

1. The process of producing a precision etched pattern on a metal surface which comprises coating a silk screen with a silicone, placing the silk screen over the metal to be etched, selectively depositing an acid resistant resin on a portion only of the metal surface through said silk screen and etching that portion of the metal surface that is unprotected by the resin with a strong acid.

2. The process of producing a precision etched pattern on a metal surface which comprises placing a silk screen over the surface of the metal to be etched, selectively depositing upon the metal surface a potentially thermosetting epoxy type resin mixed with small amounts of silicone through said silk screen, curing the resin to form an acid resistant coating on selected portions of the metal surface and etching that portion of the metal that is unprotected by the cured resin with a strong acid.

3. The process of producing a precision etched pattern on a metal surface which comprises coating a silk screen with a silicone, placing the silk screen over the metal surface to be etched, selectively depositing a potentially thermosetting epoxy type resin mixed with small amounts of silicone on the metal surface through said silk screen, curing the resin to form an acid resistant coating on selected portions of the metal surface, etching that portion of the metal that is unprotected with a strong acid.

References Cited in the file of this patent UNITED STATES PATENTS 2,107,294 Griswold Feb. 8, 1938 2,535,794 Hempel Dec. 26, 1954 2,699,424 Nieter Jan. 11, 1955 OTHER REFERENCES Dow Corning Silicanes, pub. by Dow Corning Corp, 1947, page 7.

British Plastics, November 1948, pages 521-527.

The Silicone Story, publ. by General Electric, July 1952, pages 5 and 6.

Modern Plastics, September 1954, pages 155-157, 160,

161 and 240442. 

1. THE PROCESS OF PRODUCING A PRECISION ETCHED PATTERN ON A METAL SURFACE WHICH COMPRISES COATING A SILK SCREEN WITH A SILICONE, PLACING THE SILK SCREEN OVER THE METAL TO BE ETCHED, SELECTIVELY DEPOSITING AN ACID RESISTANT RESIN ON A PORTION ONLY OF THE METAL SURFACE THROUGH SAID SILK SCREEN AND ETCHING THAT PORTION OF THE METAL SURFACE THAT IS UNPROTECTED BY THE RESIN WITH A STRONG ACID. 